Perpendicular magnetic recording (PMR) has been developed in part to achieve higher recording density than is realized with longitudinal magnetic recording (LMR) devices and is believed to be the successor of LMR for next generation magnetic data storage products and beyond. A single pole writer combined with a soft magnetic underlayer has the intrinsic advantage of delivering higher write field than LMR heads. A conventional PMR write head as depicted in FIG. 1 typically has a main pole layer 10 or write pole with a write pole tip 10t at an air bearing surface (ABS) 5 and a flux return pole (opposing pole) 8 which is magnetically coupled to the write pole through a trailing shield 7. Magnetic flux in the write pole layer 10 is generated by coils 6 and passes through the pole tip into a magnetic recording media 4 and then back to the write head by entering the flux return pole 8. The write pole concentrates magnetic flux so that the magnetic field at the write pole tip 10t at the ABS is high enough to switch magnetizations in the recording media 4. A trailing shield is added to improve the field gradient in the down-track direction.
Referring to FIG. 2, a top view is shown of a typical main pole layer 10 that has a large, wide portion called a yoke 10m and a narrow rectangular portion 10p called a write pole that extends a neck height (NH) distance y from the ABS plane 5-5 to a plane 3-3 parallel to the ABS where the write pole intersects the yoke at the neck 12. The main pole layer 10 flares outward at an angle θ from a dashed line 11 that is an extension of one of the long rectangular sides of the pole 10p. PMR technologies require the write pole 10p at the ABS to have a beveled shape so that the skew related writing errors can be suppressed. Note that the top surface of the main pole layer 10 has sides 10t while the bottom surface has sides 10b. In other words, the top surface outlined by sides 10t has a larger surface area than the bottom surface outlined by sides 10b and from a top-down view, the sides 10t do not overlap the sides 10b. 
In the fabrication process, the yoke 10m and write pole 10p may be formed by patterning a photoresist layer (not shown) above an alumina layer and then transferring the pattern through the alumina by an etching process to form a mold. An electroplating method is used to deposit a main pole layer 10 that fills the cavity in the alumina. Finally, a lapping process is employed to remove the end of the write pole 10p opposite the yoke 10m and thereby define an ABS plane 5-5.
To achieve high areal recording density with PMR technology, key requirements for the PMR writer design are to provide large field magnitude and high field gradient in both down-track and cross-track directions. In practice, these two requirements are often traded off with each other to balance the overall performance. One approach involves optimizing the geometry of the main pole layer such as modifying the values for NH and flare angle θ. A short NH or large θ can increase write field magnitude effectively. However, too short of a NH leads to problems of meeting process tolerance during manufacturing while too large of a flare angle θ may cause a large amount of adjacent track erasure because of a large fringe field. In today's commercial PMR writer products, NH is generally greater than 0.1 micron and flare angle θ is kept less than 45 degrees.
In order to pattern the beveled main pole (MP) comprising yoke 10m and write pole 10p, several different techniques have been adopted by the thin film head industry and include high tilt ion milling and electroplating over a photoresist with beveled sidewall or beveled alumina mold etched by a reactive ion etch (RIE). The write pole 10p is where a beveled sidewall is required for optimum performance while the yoke 10m does not need a beveled sidewall. However, during MP fabrication, the write pole and yoke are subjected to the same process conditions and as a result, the side wall at the write pole 10p and the sidewall at the yoke 10m both have a bevel angle (BA).
Referring to FIG. 3, a three dimensional view of the main pole layer 10 from FIG. 2 is illustrated from an oblique angle in order to show the bevel angle α which is typically from 5° to 12° and around the perimeter of the yoke 10m and write pole 10p and at the neck 12. The main pole layer has sides or sloping sidewalls 10s with a sidewall top 10t and a sidewall bottom 10b and the write pole tip 10r is formed at the ABS. Note that the amount of overhang g of a sidewall top 10t over a sidewall bottom 10b is equal to the bevel angle α×the main pole layer 10 (or alumina layer) thickness f. This beveled design invariably reduces the volume of magnetic material behind the ABS and especially in the yoke 10m adjacent to the neck 12 where a beveled sidewall is not necessary. As a result, the writability of the main pole layer 10 is significantly reduced since the volume of magnetic material is most critical near the neck 12.
Referring to FIG. 4, a top-down view of an alumina layer 21 with sidewalls 21s is shown after a typical photoresist patterning and RIE process that uncovers substrate 20 and prior to electroplating the main pole layer. Note that the sidewall tops 21t begin to flare outward (away from a vertical axis) near the plane 22-22 while the sidewall bottoms 21b begin to flare outward at the plane 23-23. Ideally, sidewall tops 21t and sidewall bottoms 21b should both diverge from a vertical axis near plane 23-23. Due to the nature of the RIE process, the bevel angle is smaller (steeper sidewall 21s) at the narrow region 20n where the write pole 10p will subsequently be formed, and is larger (shallower sidewall 21s) adjacent to the open region 20 where the yoke 10m will later be placed. Thus, from a top-down view, the sidewall 21s is wider near the large opening 20 and narrower adjacent to the narrow opening 20n. This problem is called positive dBA (differential bevel angle) which means the yoke 10m (FIG. 3) will be undercut to a more tapered angle α than the write pole 10p. As a result, there is a significant reduction in the volume of magnetic materials that are plated in the yoke 10m adjacent to the neck 12 compared with the desired situation where the sidewalls 10s in the yoke 10m are vertical. Accordingly, there is a need to implement a main pole layer fabrication process that will enable the sidewalls in the yoke to be essentially vertical near the neck transition point where write pole 10p adjoins yoke 10m and thereby increase main pole layer writability.
Another challenge for the PMR fabrication process is to make the neck transition as sharp as possible. In FIG. 5, a typical top view of a photoresist layer 25 that has been patterned in the shape of a main pole layer on a substrate 24 is shown. Due to an optical proximity effect, the neck corner 25b is rounded even though the mask design has sharp angles for the juncture of the side 25a and side 25c at the neck. The corner rounding problem is well known in photolithography. A common technique called OPC (optical proximity correction) involving addition of sub-resolution features (hammer heads or serifs) near the corners of features on the photomask is often used to reduce corner rounding. However, the neck transition will never be perfectly sharp since the sharpness (resolution) of the corner is dependent on exposure λ and the wavelength of advanced lithography tools is currently limited to 193 nm (ArF excimer laser).
Corner rounding in the neck region continues to impose serious problems for main pole dimensional control. For example, the NH design distance y (FIG. 2) between the ABS plane 5-5 and plane 3-3 at neck 12 is ever decreasing for higher areal density designs. A rounded corner at neck 12 effectively moves the plane 3-3 closer to the ABS and allows less room for ABS placement error. If the ABS is formed at an angle to the plane 3-3 rather than in a parallel arrangement, then the ABS may easily cut into a rounded corner near neck 12 and effect neck height distance y. In other words, any ABS position errors induced by the slider process will be translated into additional dimension errors for y.
A search of the prior art revealed the following references. In U.S. Patent Application Publication No. 2001/0028531 and in related U.S. Pat. No. 7,190,553, a magnetic pole layer is shown with write pole, yoke, and neck portions having inclined sidewalls that are sloping inward.
U.S. Patent Application Publication No. 2007/0014048 teaches a two step etching sequence involving a first slit pattern and a second slit pattern to form an alumina mold so that a subsequently deposited main magnetic pole layer has enhanced volume whereby the overwrite characteristic can be improved.
In U.S. Patent Application Publication No. 2006/0002021, a process is provided for forming a main pole layer wherein a bottom yoke portion and a write pole are deposited first and then a top yoke portion is formed on the bottom yoke portion.
U.S. Patent Application Publication No. 2006/0077590 discloses a method of forming a main pole layer wherein a pole encasing layer is formed on a substrate that can serve as an electrode for a plating operation. A groove is etched in the pole encasing layer by means of a BCl3, Cl2, and CF4 gas mixture to produce a bevel angle between 5 and 12 degrees.
In U.S. Patent Application Publication No. 2006/0276039, an improved mold for forming a PMR write head is fabricated by forming a Ta layer in the intended yoke region and then depositing alumina on the substrate. After polishing, the alumina layer is coplanar with the Ta layer. During the etching process to form a mold opening, sloping sidewalls are formed in the pole region of the opening in the alumina layer and vertical sidewalls are formed in the yoke region of the opening in the Ta layer.